Variable stroke property engines comprising a plurality of links provided between the piston and crankshaft for varying the stroke of the piston by moving an end of one of the links that is connected to the engine main body are proposed in Japanese patent laid open publications Nos. 9-228858, 2004-138229 and 2004-150353.
It is known that the resistance to the engine increases if the crankshaft contacts the lubricating oil in the oil pan. Particularly in the case of a variable stroke property engine, because there are a large number of moving parts such as links that move about within the crankcase, the contact between the lubricating oil and such moveable parts is more problematic that that in more conventional fixed stroke engines in which the piston and crankshaft are connected to each other simply by a connecting rod (single link).
It is also know to provide a baffle plate above the oil surface of the lubricating oil to control the disturbance in the oil surface and prevent excessive splashing of lubricating oil onto the crankshaft. However, in the case of an engine incorporated with a variable stroke piston mechanism including a plurality of links, providing a special baffle plate for preventing the splashing of lubricating oil onto the variable stroke piston mechanism leads to an increase in the number of component parts and the amount of manufacturing work.
Also, the cylinder block of an engine is provided with passage for communicating the interior of the cylinder head with the interior of the crankcase for the purpose of returning the lubricating oil that has lubricated the valve actuating mechanism to the oil pan and to reduce the pressure pulsation in the crankcase.
In a conventional fixed stroke engine using only a connecting rod (single link) to connect the piston with the crankshaft, it is possible to avoid the return lubricating oil and blowby gas from contacting the crankshaft simply by forming a passage in a peripheral part of the cylinder. However, in a variable stroke property engine, because the variable piston stroke mechanism includes a large number of moving parts including a plurality of links, it is difficult to prevent the lubricating oil from dripping onto the links and thereby increasing the resistance to the movement or from obstructing the flow of the blowby gas. In particular, because the second link which connects the first link with the crankshaft moves at a high speed, splashing or dripping of lubricating oil onto the second link has a significant effect on the resistance to the movement thereof.